You gotta remember that the Shark wasn't Ax's first morph.
Which I stated.
Like Gumrod suggests, this pretty much refers to their "on screen" morphs.
Similarly, Aldrea's first "on screen" morph is a Chadoo, which symbolizes her willingness to take part and explore new cultures rather than shun them arrogantly or let her negative experiences dominate her over all world perspective.
I think it may work better for the human Animorphs than for Ax, though. I guess I always thought of sharks as pretty simple creatures; they're smart, but they're single-minded. I don't think that describes Ax for very long.
Ah, but consider Ax's actions. Yes, he declares Jake his Prince in book 4, but he doesn't even trust Jake with his knowledge until book 8. Then in The Decision Ax immediately drops his loyalty to Jake as soon as he's on an Andalite ship. You could argue that was just a minor set back, but it's what Ax did when the cards were on the table. Sharks have a very primitive mindset, which tells them to go with the thing that makes you live longer. Ax's move in that moment wasn't so different.
And while we're on the subject of the Shark's instinct, what happens when you make food available to a shark? Frenzy.
Even fifteen books later (if you count MM1) Ax still acts like an out of control maniac when food is presented to him. He goes wild on cinnamon buns, chilli, and on the food at Jake's house during book six (making Jake's parent's take him to a shrink, if I recall). He can slap a Z space communicator together, but he doesn't learn to control himself very well when food is presented to him. Again, pretty primitive, shark-like behavior, even if he doesn't always act like that.
What does this make their battle morphs then?
Jake, Tiger
Neither a leader or a follower. Tigers are not pack or pride animals by nature. But their cunning and strength make them animals to be respected as well as healthily feared. Jake's tenacity and his ability to focus on the main goal, while always being aware and respectful of his surroundings are indicative of the tiger's nature.
Rachel, Grizzly Bear
Let's face it, the Grizzly Bear is Rachel's true battle morph. The elephant represented a kind of clumsy eagerness whereas the Grizzly represented that same strength, only tempered with experience. It's like the difference between a blunt club made of stone and a sharp sword, folded and tempered over time with skill and patience.
Tobias, Hawk
As a bullied child, Tobias had to become aware of what was in front of him. Like the hawk, which hunts from a safe distance, Tobias never makes a move until he's at least 80% certain of a safe and clean kill.
Cassie, Wolf
The Wolf is a balance of strength and power. It can only do damage with its claws and teeth, but these are enough to make the enemy or target think twice. We know Cassie deliberately chooses the wolf morph so that she can do the least amount of damage in an attack. The wolf is as symblic of her respect for life and nature as the horse is.
Marco, Gorilla
Once again, the Gorilla is simply the best of Marco. Even in battle he never holds back who or what he is.